In a membrane separation activated sludge treatment method that is of a typical example to which the apparatus of the invention is applied, for example, an organic material contained in industrial wastage or living drainage or drainage containing microorganisms or bacteria is biologically treated, solid-liquid separation is performed using various separation membranes, and treated water is recovered in a treated water tank or discharged. A usual membrane separation activated sludge treatment apparatus includes a raw water regulating tank, a denitrification tank, and an aeration tank. In the raw water regulating tank, a liquid level in the tank is measured with a liquid-level meter, and a liquid sending pump is intermittently driven to adjust a liquid-level height in the tank within a predetermined range. After raw water sent by a liquid sending pump is introduced to the denitrification tank, the raw water overflowing from the denitrification tank is caused to flow in the aeration tank. A membrane filtration unit is immersed in the aeration tank. The membrane filtration unit performs the membrane separation of the raw water into treated water and a contaminant of activated sludge, the filtrated treated water is sucked by a suction pump, and the treated water is recovered in the treated water tank or discharged. A large proportion of the excess sludge in the aeration tank is sent to a sludge storage tank and incinerated after dried. Part of the excess sludge is sent back to the denitrification tank by the liquid sending pump to circulate between the denitrification tank and the aeration tank.
A typical example of the membrane filtration unit includes a hollow fiber membrane module and an air diffuser apparatus. A plurality of sheet-like hollow fiber membrane elements are arranged at predetermined intervals in the hollow fiber membrane module, and many pieces of porous hollow fiber are arranged in parallel in the same plane in the hollow fiber membrane element. The air diffuser apparatus is disposed below the hollow fiber membrane module. A whole shape of the hollow fiber membrane module including the plurality of hollow fiber membrane elements is formed into a substantially rectangular solid. In the air diffuser apparatus, a plurality of air diffusion pipes are provided in parallel, and one end of each air diffusion pipe is connected to air supply piping of an aeration blower. The air diffusion pipe is made of metal or resin, and holes or slits are provided in the air diffusion pipe. The air delivered from the aeration blower is discharged in the sludge through the air diffuser apparatus.
The air discharged from the air diffuser apparatus becomes a bubble and rises, and the air and the surrounding sludge are mixed to become a gas-liquid mixing flow, thereby swinging the hollow fiber membrane element of the hollow fiber membrane module located above. The sludge adhering to a membrane surface of the hollow fiber membrane element is peeled off from the membrane surface by the swing of the hollow fiber membrane element to perform so-called air scrubbing cleaning that suppresses clogging of the membrane surface. When the living drainage or plant wastage is treated, air generated from the air diffuser apparatus is brought into contact with the organic material in the sludge of the aeration tank under the existence of the aerobic microorganisms, and the organic material is absorbed and metabolism-resolved by the aerobic microorganisms, thereby performing the biological sludge treatment.
The hollow fiber membrane module and the air diffuser apparatus are surrounded by a rectangular tubular closure plate whose upper and lower ends are opened. A gas-liquid mixing flow is generated by rise of a bubble generated from the air diffuser apparatus, and the closure plate is used as a wall portion that guides the gas-liquid mixing flow from an upward flow to a downward flow. The gas-liquid mixing flow that generated by the bubble discharged from the air diffuser apparatus is not dispersed in an oblique direction, but the gas-liquid mixing flow rises straight to efficiently come into contact with the hollow fiber membrane module. At this point, the hollow fiber membrane is swung by the even dispersion of the gas-liquid mixing flow with respect to the membrane surface of the hollow fiber membrane module to evenly clean each hollow fiber membrane element. Then, the gas-liquid mixing flow flows to the surroundings beyond an upper-end portion of the closure plate, and the gas-liquid mixing flow comes down to form a vertical swirling flow as a whole. The activated sludge is stirred by the swirling flow to homogenize the biological treatment.
A membrane separation module including a filtration membrane having a plurality of fine holes is also used in addition to the sheet-like hollow fiber membrane element including the porous hollow fiber is used as the component. For example, there are various known separation membranes such as a planar membrane type, a tubular membrane type, and a saclike membrane type. Examples of the material for the separation membrane include cellulose, polyolefin, polysulphone, PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), and ceramics. The hollow fiber membrane module in which the hollow fiber membrane element is utilized is frequently used because of a wide filtration area.
As to an average diameter of the fine hole of the porous hollow fiber, generally the average diameter ranges from 0.001 to 0.1 μm in a generally-called ultrafiltration membrane, and the average diameter ranges from 0.1 to 1 μm in a generally-called microfiltration membrane. For example, preferably the hole diameter is equal to or less than 0.5 μm when the membrane separation module is used in solid-liquid separation of the activated sludge, and preferably the hole diameter is equal to or less than 0.1 μm when sterile filtration is required like water purification.